The invention relates to mobile wireless communication networks in which the communication channels are partitioned into overlay channels that are potentially accessible by any mobile station (within a pertinent cell), and underlay channels that are accessible only by mobile stations having relatively low path loss. More particularly, the invention relates to methods for determining how the channels are to be partitioned, and for setting the criterion that identifies those mobile stations that may have access to underlay channels.
In the operation of mobile wireless communication networks, the danger of interference between cells is one factor that limits the number of cells in which a given communication channel (e.g., a frequency channel) can be used. However, increasing the number of cells in which a given channel is used is advantageous, because it allows the total available radio-frequency spectrum to be used more efficiently, and thus increases the total information-carrying capacity of the network.
Practitioners in the art of wireless communication have recognized that some of the mobile stations within a given cell are less prone to cell-to-cell interference then others. More specifically, those mobile stations that suffer relatively small path loss to or from their (currently serving) base station can communicate with such base station with a concomitantly low danger of interference with neighboring base stations, provided that the path loss to the neighboring base stations is relatively high.
Practitioners in the art have further recognized that by giving special treatment to such mobile stations, it is possible to increase the capacity of the network. Such an approach to capacity enhancement is sometimes referred to as xe2x80x9creuse partitioning.xe2x80x9d A network that is designed to give such special treatment to a class of mobile stations is here referred to as an underlay-overlay (U-O) network.xe2x80x9d
In a U-O network, each cell includes an overlay (or outer cell) region, and an underlay (or inner cell) region. The underlay is typically distinguished from the overlay by having smaller path loss between mobile stations and the serving base station. The available channels are partitioned into two groups, which we refer to as A-channels and B-channels. The A-channels are assigned to the overlay, but are potentially accessible by all mobile stations. The B-channels are accessible only to mobile stations in the underlay. (Mobile stations in the overlay and underlay are referred to, respectively, as A-mobiles and B-mobiles.)
Each of the B-mobiles has a path loss to the serving base station that is less than a threshold Pi, where the index i, i=1, . . . , N identifies a given one of the N cells in the network. The A-mobiles, which occupy the overlay, have higher path losses. Because the B-mobiles have lower path losses, the B-channels can be reused more frequently than the A-channels.
Typically, power control is applied between the mobile stations and the serving base station. One example of power control is constant received power control, in which the transmitted power in the uplink (and in some cases, also on the downlink), is regulated to compensate for the path loss to the receiving station. In such a case, at least the uplink transmissions between the base station and the B-mobiles will have lower power than those between the base station and the A-mobiles. This tends to further reduce interference throughout the network, and thus to extend the scope and effectiveness of the overlay.
The proportion of a cell""s traffic that can be supported within the underlay is referred to as the absorption, which we represent below by the symbol xcex1. The pertinent traffic is made up of those mobile stations that have relatively low path loss to their serving base station, and relatively high path loss to neighboring base stations. Such mobile stations, as noted, are referred to as the B-mobiles. The network capacity is increased by permitting the B-mobiles to reuse channels more frequently than the A-mobiles. The proportion of mobile stations that are B-mobiles will depend upon the actual level of channel reuse, the propagation characteristics between cells in the pertinent part of the network, and the actual geographical distribution of mobile stations (at a given time).
The efficacy of a U-O network is sensitive to the particular scheme used for allocating channels between the underlay and overlay networks. The simplest scheme is to make a fixed allocation. However, this scheme loses some trunking efficiency. Trunking efficiency is the gain in network capacity that is obtained by sharing channels freely in order to accommodate fluctuations in traffic. As a consequence, the fixed allocation scheme tends to reduce the gains in performance otherwise achievable through the increased frequency of channel reuse in the underlay.
There has remained a need for an allocation method that maximizes, or nearly maximizes, the capacity gains potentially available from the U-O approach.
We have devised such an allocation method. In accordance with our new method, underlay calls are permitted to overflow onto overlay channels. Moreover, the boundaries of the underlay cells are chosen adaptively, in such a way that the risk of excessive interference is constrained within an acceptable level. In this way, network resources can be used efficiently in the presence of the inhomogeneities in traffic and in signal propagation characteristics that are typical of real networks. Within each cell, the underlay boundary is defined by a respective path-loss threshold. In this context, we use the term design (of U-O networks) to include, inter alia, the allocation of channels between the overlay and the underlay, and the setting of the underlay boundaries.
In a broad aspect, our method involves obtaining a blocking curve for each cell. The blocking curve expresses the manner in which the average blocking (i.e., the refusal to accept an offered call) depends upon the absorption in the given cell. The blocking curve may be based upon a theoretical, or a partially empirical, model of cell traffic. Parameters that affect the blocking curve will typically include the total spectrum (i.e., the total available number of channels), the channel split, and the rate of traffic offered to the given cell.
In a typical blocking curve, the blocking decreases as the absorption increases. However, there is generally some value of absorption above which the rate of decrease is relatively small. Thus, such a blocking curve (as shown, e.g., in FIG. 7) can be used to establish an operating range, beginning at the smallest absorption value sufficient to meet a target level of blocking performance, and ending at an absorption value above which the rate of further improvement is relatively small.
For each cell, a respective approximation, such as a linear approximation, is chosen to represent such cell""s blocking behavior within the operating range. There is then defined an objective function for the overall network, based on the respective single-cell approximations. Optimization of this objective function is equivalent to maximization of network capacity.
From path-loss measurements for each cell, there is determined a parametric dependence of absorption upon the path-loss-threshold Pi. The path-loss thresholds at the ends of the operating range are also determined. This enables the objective function to be treated as a function of the path-loss thresholds, and enables the operating range to be defined in terms of the path-loss thresholds.
The objective function is optimized over the path-loss thresholds. This optimization is subject to the constraint that each path-loss threshold must lie within its respective operating range, and to the constraint that the risk of unacceptable interference must be less than a stated level.
The result of the optimization procedure is a set of optimal values for the respective path-loss thresholds. Each of these path-loss thresholds defines the underlay boundary for its corresponding cell.
Typically, there will initially be many potential channel splits. However, not all of these channel splits will be feasible. For example, some channel splits may provide insufficient underlay channels to meet the target level of blocking performance. As a further example, some channel splits may fail to satisfy the interference constraint for one or more cells. Accordingly, certain embodiments of the invention include a step of pruning the channel splits, leaving only those channel splits that are feasible in view of pertinent constraints. The optimization of the objective function is separately carried out for each of the feasible channel splits. The overall network blocking is computed for each of the resulting optimal solutions. For the ultimate network design, that channel split is selected that gives the least overall network blocking.